Growing research and development efforts are being made for faster devices for use in electronic systems with an emphasis put on heterojunction bipolar transistors which can provide larger current driving capabilities in comparison with a field effect transistor. On of the heterojunction bipolar transistors of the type having a compositionally grade base region is disclosed by J. R. Hayes et al. in "BIPOLAR TRANSISTOR WITH GRADEDBAND-GAP BASE", ELECTRONIC LETTERS, 26th May 1983, vol. 19, No. 11pages 410 to 411. The heterojunction bipolar transistor disclosed by Hayes et al. has a base region compositionally graded from Al.sub.0.15 Ga.sub.0.85 As to GaAs which results in an energy band diagram similar to that illustrated in FIG. 1 of the drawings. The energy band illustrated in FIG. 1 can be divided into three sections corresponding to the emitter region, the compositionally graded base region and the collector region, respectively. The first section corresponding to the emitter region has a relatively wide bandgap, and the second section corresponding to the compositionally graded base region has a graded bandgap. Namely, the graded bandgap has at one end thereof a relatively wide bandgap allowing the second section to merge into the first section and at the other end thereof a relatively narrow bandgap allowing the second section to merge into the third section as will be seen from FIG. 1. The compositionally graded base region produces a quasi-electric field which accelerates minority carriers 1 injected from the emitter region into the compositionally graded base, and, for this reason, the minority carriers 1 are expected to pass the compositionally graded base region at an ultra-high speed. In the energy band diagram shown in FIG. 1, reference numerals 2 and 3 designate the bottom of the conduction band and the top of the valence band, respectively, and the Fermi-levels of the emitter region, the compositionally graded base region and the collector region are indicated by reference numerals 4, 5 and 6, respectively.
However, a problem is encountered in the prior-art heterojunction bipolar transistor with the compositionally graded base region in inter-valley scattering. In detail, when the base region is compositionally graded from Al.sub.0.15 Ga.sub.0.85 As to GaAs, this compositional grading corresponds to an electric field of about 10 kV/cm on the assumption that the compositionally graded base has a thickness of about 150 nano-meters. This electric field results in that a large amount of inter-valley scatterings take palce in the conduction band. As a result, the minority fell short of expectation in speed up.
One of the approaches to solve the problem in inter-valley scattering is to adopt an abrupt emitter-base heterojunction which allows the injected minority carriers to move in a ballistic or near-ballistic manner. A typical example of the heterojunction bipolar transistor with the near-ballistic operation is disclosed by D. Ankri et al. in "HIGH-SPEED GaAlAsGaAs HETEROJUNCTION BIPOLAR TRANSISTORS WITH NEAR-BALLISTIC OPERATION", ELECTRONIC LETTERS, 17th Feb. 1983, Vol. 19, No. 4, pages 147 to 149. The heterojunction bipolar transistor of the type providing the ballistic or near-ballistic operation has an energy band illustrated in FIG. 2 of the drawings. In the energy band diagram, reference numerals 11 and 12 designate the bottom of the conduction band and the top of the valence band, respecitvely, and the Fermi levels of the emitter region, the base region and the collector region are denoted by reference numerals 13, 14 and 15, respectively. As will be seen from FIG. 2, the emitter region is smaller in electron affinity than the base region but is wider in bandgap than the base region. These emitter and base regions results in a hetrojunction with an abrupt potential discontinuity 16, and the abrupt potential discontinuity 16 provides a kinetic energy corresponding to the potential gap to minority carriers, or electrons, injected from the emitter region into the base region, thereby accelerating the electrons to move in the ballistic manner.
However, another problem is encountered in the prior-art heterojunction bipolar transistor with ballistic operation in that each of the electrons terminates the ballistic movement in the initial stage of traveling over the base region. This is because of the fact that each of the electrons loses the kinetic energy at around the mean free path which is on the order of 40 nano-meters in a p.sup.+ -type gallium-arsenide. However, a heterojunction bipolar transistor has a base region with a thickness greater than a hundred nano-meters, so that each of the injected minority carriers are diffused in the remaining base region after termination of the ballistic movement. In order to overcome the above problem, it may be proposed to decrease the thickness of the base region within the mean free path, however the base region with a thickness of about 40 to 50 nano-meters results in a extreme large base resistance which gives rise to deterioration in device characteristics. Thus, the heterojunction bipolar transistor with the ballistic operation currently does not provide the perfect solution for the speed up.